For an oncoming train in dim light, the bigger issue is often technique and available light, not just the camera body. When AF starts hunting, a reliable approach is to pre-focus on a spot where the train will be, then wait for it to reach that point. Manual focus can work well here too.

Stopping down can give more depth of field, which helps with slight focus errors, but only if you still have enough shutter speed. Also check whether the softness is actually motion blur rather than missed focus: if shutter speed is too slow, the train can look blurry even when focus is correct. In that case, use a wider aperture and/or raise ISO to keep shutter speed up.

Burst shooting also makes timing less precise, since you’re limited by the camera’s frame rate. A single well-timed shot at a pre-focused point is often sharper than a sequence.

A remote trigger system can also be used in controlled situations, but safety around tracks is critical and local laws must be respected.

So: if AF is unreliable, switch to pre-focus or manual focus, prioritize sufficient shutter speed, and use more depth of field when light allows.